1. Technical Field
The present invention relates to a light-emitting element, a light-emitting apparatus, a display apparatus, and an electronic device.
2. Related Art
Organic electroluminescent elements (organic EL elements) are light-emitting elements that include at least one organic light-emitting layer between an anode and a cathode. Upon the application of an electric field between the cathode and the anode, electrons in the cathode and positive holes in the anode are transferred to the light-emitting layer and recombine with each other in the light-emitting layer, generating excitons. When excitons return to the ground state, excitons release the excitation energy as light.
One example of such light-emitting elements includes three light-emitting layers corresponding to red (R), green (G), and blue (B) between a cathode and an anode and emits white light (see, for example, JP-A-2005-100921). Such a white-light-emitting element can be used in combination with a red (R), green (G), or blue (B) color filter in each pixel to display full-color images.
A light-emitting element according to JPA-2005-100921 further includes an intermediate layer formed of a hole-transport material between light-emitting layers. The intermediate layer functions to restrict electron transfer from one light-emitting layer on the cathode side to another light-emitting layer on the anode side. The light-emitting element can therefore emit white light at a predetermined driving voltage.
However, with a small electric current between the cathode and the anode, balance is lost between the three light-emitting layers of the light-emitting element according to JP-A-2005-100921, and the three light-emitting layers consequently cannot emit white light. With a top emission structure, since light is extracted utilizing optical resonance, white light within a light-emitting element can be extracted substantially as red, green, and blue light. However, white light having an irregular waveform produces red, green, and blue light having an irregular waveform, displaying undesired colors.
For example, although the light-emitting element according to JP-A-2005-100921 can slightly emit light even with a weak current such as a dark current, the color of light is different from white. Thus, a display apparatus that includes this light-emitting element has a reduced contrast (poor black reproduction).